Balanced transformer



April 24,'1962 Filed June 24, 1959 w. M. MURPHY, JR

BALANCED TRANSFORMER 2 Sheets-Sheet 1 i? P19/0R ART i Aflomey April 24, 1962 w, M. MURPHY, JR 3,031,609

BALANCED TRANSFORMER Filed June 24, 1959 2 Sheets-Sheet 2 w .A mw w. R. .w 'im m L m ,m V T. mm M vM Y B .NN l f Sac. h 22:30 oooJ United States Patenti() 3,031,609 BALAN CED TRANSFORMER William M. Murphy, Jr., Wellesley Hills, Mass., assiguor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed .lune 24, 1959, Ser. No. 822,702 1 Claim. (Cl. 323-43) This invention relates to transformers and more particularly to capacitive balanced transformers.

One object of the invention is to provide a transformer wherein all of the principal interwinding capacitances are substantially equalized and shunted to ground.

Another object of the invention is to eliminate the use of winding shields or lthe like in a transformer.

In conformity with these objects, the preferred embodiment of the invention is characterized by an elongated core member having a primary winding thereon together with a pair of opposed and axially spaced secondary windings. The two secondary windings are connected in series and center-tapped to ground and one of the secondary windings is reversely wound. All three of the windings are Wound from startto finish on the core in the same axial direction. This winding configuration together with the connective arrangement of the windings substantially equalizes all of the principal interwinding capacitances of the transformer and shunts these interwinding capacitances to ground.

These and other objects of the present invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a transformer showing the interwinding capacitances between the windings thereof; v

FIG. 2 is a vectorial representation of the interwinding capacitances shown in FIG. l;

FIG. 3 is a vectorial representation of the interwinding capacitances in a transformer constructed inaccordance with the present invention;

FIG. 4 is a schematic illustration of a modulator utilizing the transformer of the present invention;

FIG. 5 is a schematic illustration and wiring diagram of the preferred embodiment of the invention; and

FIG. 6 is another schematic illustration of the preferred embodiment of the invention.

Referring more particularly to FIG. 1, there is shown a conventional three winding or three pi-type transformer including a core means orV core 10 having a primary winding 12 thereon together with a pair of series connected secondary windings 14 and 16. The secondary windings 14 and 16 are positioned on opposite sides of the primary winding 12 and are axially spaced along the core 10 a predetermined distance from the primary winding. All three windings are wound in the same direction (either clockwise or counter-clockwise) and in being applied to the core 10 are Wound from start (S) to (F) finish in the same direction axially of the core. With a transformer so constructed and so wound, the principal interwinding and stray capacities or capacitances can be represented by The components of the transformer shown in FIG. 1 together with the interwinding capacitances can be vectorially represented as shown in FIG. 2 wherein P represents primary winding 12, S1 represents secondary winding 14 and S2 represents primary winding 16. With the secondary winding S1 not connected to the ground plane, the interwinding capacitances are clearly unbalanced.

By reverse Winding one o-f the secondary windings and changing the ground and center tap connections, the transformer of FIG. 2 may be illustrated vectorially as shown vin FIG. 3, which vectorial illustration is illustrative and exemplary of the present invention. An examination of FIG. 3 will reveal that both secondary windings S1 and S2 are connected to the ground plane with winding S2 being reversed in direction and that the more troublesome interwinding capacities are shunted to ground. The capacitances c2 and c6 in particular, going from (S) to (F) and being unequal in magnitude, are in general negligibly small. This minuteness of c2 and c6 results from the fact that the area of the capacitor plate for (F) is much larger than Ythe area for (S) and that the distances between them are relatively large. With the interwinding and stray capacitances being thus distributed and shunted to ground, it will readily be appreciated that winding shields such as Faraday shields or the like would not be necessary.

Referring now to FIGS. 5 and 6 and the preferred embodiment of the invention, it can readily be seen (FIG. 5) that all three windings P, S1 and S2 of the transformer are connected to a ground in the form of metal housing or the like 18 (shown in phantom). The primary windingP extends between points 3 and 4 with the (F) or finish end of the winding being connected to ground. The two secondary windings S1 and S2 are connected in series and The (F) or nish ends of the two secondary windings, points 2 and 6 constitute the output of the transformer secondary.

It will be especially noted in FIGS. 5 and 6 that the primary winding P and the secondary ywinding S1 are both wound in the same direction while the secondary winding S2 is wound in the opposite direction. It is also significant to note that all of the windings are wound from start (S) to (F) iinish in the same direction, i.e., axially of the core in one direction in FIG. 6 or from top (S) to bottom (F) in FIG. 5.

In this preferred embodiment of the invention, all of the windings are wound with No. 41 S.S.E. wire with the primary winding being comprised of 330 turns and both of the secondary windings being comprised of turns apiece. Using No. 41 S.S.E. wire, the windings will be relatively small, the primary P having a thickness of approximately ls and a maximum outside diameter of approximately 0.355. The secondary windings will have a thickness of approximately 1/16" and of course will be much smaller in diameter than the primary winding P. Y The'three windings are wound upon core 10 which in this instance has a diameter of approximately 0.145" and a length of approximately 0.450. The electrical properties of the three windings in the preferred embodiment of the invention should measure substantially as follows:

Winding Inductance D.C. Resistance, ohms P to ground 1 86 millihenrips 29. 3 S1 to ground 2.97 microhenries 8. 6 Si to ground 3.02 mierohenries 8. 8

Patented Apr. 24, 1962Y nent in a balanced modulator generally indicated by the reference numeral 20. The primary P of the transformer is connected to a local oscillator 22 and both the primary winding P and secondary windings S1 and S2 are grounded to a metallic housing or the like 24 (shown in phantom) which encases the modulator. The secondary windings S1 and S2 are center-tapped to ground and are connected to -a pair of reversely positioned diodes 26 and 28 which in this instance take the form of a pair of CK606 diodes. A balance potentiometer 30 is connected across the two diodes together with a pairof identical swamping resistors 32 and 34 each having in this particular embodiment of the modulator a value of 8 thousand ohms. A Variable capacitor 36 and a fixed capacitor 38 lare also connected to ground across the two diodes, variable capacitor 36 having -a capacity Varying between 1.5-8 microfarads and capacitor 38 having a capacity o-f 5 microfarads. The output of modulator 20 is taken across terminals 44 while the video input signal is fed into the modulator at terminal `42 and across resistor 40, resistor 40 having in this instance a value of 4700 ohms.

Utilizing the balanced transformer of the present invention, the performance of the modulator 20 was obtained experimentally as follows: In determining the eiliciency of the modulator 20, the modulator output terminals 44 were connected in standard fashion to a 155 kc. IF stage, i.e. output through a 100-micromicrofarad capacitor to the center tap of a standard B-2 I-F coil. The plate tuned circuit of the IF stage was loaded down with a 10,000 ohm resistor. The gain of the IF stage was measured with a 600l ohm signal generator connected from grid to ground.

The overall 155 kc. gain was then measured by feeding in the 155 kc. signal at the modulator video input terminal 42. The gain over that which was measured with the 600 ohm signal generator was approximately 6 decibels as expected due to the 2:1 step up of the grid coil connection.

The overall conversion gain was measured by feeding in a 15 kc. video signal and ya 7.0 v. R.M.S. local oscillator drive signal at 170 kc. The difference in the gains as measured in feeding in the 155 kc. and 15 kc. video signals yielded the efliciency in decibels. The maximum ehiciency as measured using this method was found to be minus 11 decibels. This value of the efficiency was maximized in testing modulator 20 by selection of the proper input resistance value. The optimum value of resistance was found to be 4700 ohms.

. The modulator 20 was also tested for balance or the ratio of a 1 v. R.M.S. video input signal to the corresponding local oscillator feedthrough as measured at the modulator output terminals 44. This ratio or balance was obtained by using a spectrum analyzer, and adjusting the level of a 15 kc. video sideband equal to the local oscillator feedthrough and then measuring the video Yinput level required to produce the sideband. The balance for modulator 20 was found to be V75 decibels when a 15 kc. video signal was used and the local oscillator `drive was set to V7.0 v. R.M.S.

With regard to balance stability, the modulator 20 was found to be sensitive to changes in temperature, local oscillator level, filament voltage level and to a much lesser local oscillator output impedance and video generator output impedance. The balance was found to be most sensitive to change in the local oscillator level. For a i v. change, i.e. from +6 v. to +8 v. R.M.S., the balance changed $26 decibels. 'The maximum variation of balance with level was found to be 34 decibels. The system exhibited very little hysteresis with level change, i.e. balance recoverable With reciprocal level change. For a il v. change in filament Voltage with diodes in parallel, the balance change wasv 1:16 decibels. Very little hysteresisY eect was noted with filament variations.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claim the invention maybe practiced otherwise than as specifically described.

What is claimed is:

A pitype transformer comprising an elongated core; a primary winding carried by said core, the outer terminal of said primary Winding being grounded; a pair of secondary windings carried by said core on opposite sides of said primary Winding and axially spaced therefrom, said secondary windings being connected in series and grounded at their junction, the primary winding and one secondary winding being wound in one direction and the other secondary winding being wound in the opposite direction so as tot substantially equalize the interwinding capacitances of said transformer and shunt said interwinding capacitances to ground.

References Cited in the Iile of this patent UNITED STATES PATENTS 2,277,775 Mueller Mar. 3l, 1942 2,542,915 Favre Feb. 20, 1951 2,568,587 Macgeorge Sept. 18, 1951 2,815,408 Hafter Dec. 3, 1957 2,848,656 Nixon Aug. 19, 1958 2,929,017 Seaton Mar. 1.5, 1960 

